A Science 2.0 article looks at the work of researcher Michael Rose over past decades: "Over the years, Rose and his lab have bred fruit flies to live four times the life span of an average fruit fly. Reasoning from those studies, Rose has proposed that, because the life spans of fruit flies have the genetic capability to be extensively prolonged, human life can be manipulated in the same way. ... Wattiaux was a French scientist working at the University of Leuven in Belgium. His study used the same fruit flies that Rose had been working with ... Wattiaux found that when he made each new generation of fruit flies that were the offspring of old parents exclusively, the flies showed an increased life span after each generation. But Wattiaux didn't know why his fruit flies lived longer. He felt that longevity increased because of a nongenetic effect, but he didn't have any direct evidence. Rose did. Wattiaux's results, he saw, showed the importance of the force of natural selection. He believed that, because natural selection stops working at a late age and fails to eliminate genes with detrimental effects, these bad genes would not be removed by natural selection. Instead, they would accumulate. In populations that reproduce early, natural selection declines early. Alternatively, populations that are old when they reproduce will continue to be subject to powerful selection until they begin to reproduce. Thus, by allowing older flies to reproduce over generations, natural selection would continue to choose the flies that are able to breed at a later age - the fittest flies."